1. Field of the Invention
The present invention relates to post-tension concrete construction and, more particularly, to a delay anchor usable anywhere along the length of a continuous tendon to stress a portion of that tendon and permitting the portion of the tendon to temporarily terminate between adjacent concrete pour phases without requiring the adjacent concrete pour phase to be complete, but subsequently allowing the coupling of different portions of the tendon in the later concrete pour to join the portions of the tendon together to make a structurally continuous tendon.
2. Description of the Background
Post-tensioning concrete entails the use of high-strength steel strand, “tendons,” that are embedded in concrete and tensioned after the concrete hardens. Using tendons under tension creates cast-in-place and precast concrete members that have superior strength characteristics when compared to similarly sized non-prestressed members.
In unbonded post-tensioning applications, the steel tendons are first coated with a corrosion preventative friction reducing grease and then encased in a plastic sheathing before being laid into concrete forms. Most tendons have a fixed anchor on one end that is attached to the tendon and that is placed adjacent to the concrete form. The other end of the tendon, also known as the “stressing tail,” is passed loosely through a stressing anchor that is affixed to the other end of the concrete form and then extends a fixed distance past the form. After the concrete is placed, cured, and hardened to a specified strength, a hydraulic jack is attached to the stressing tail to apply tension to the tendon. In some conditions a tendon may have stressing anchors on both ends and no fixed end anchor is used.
There are numerous variations on and specialized components for post-tensioning. For example, sometimes concrete is cast in phases, with continuous tendons passing through the multiple phases. There are construction joints between the phases, and intermediate stressing is used for the tendons located at construction joints between phases so that the tendons in separate phases can be tensioned separately and the formwork below each phase removed after it has been tensioned.
After one section of concrete is placed, cured, and hardened to a specified strength in its formwork, a hydraulic jack is attached at some intermediate point along the tendon to apply tension to the tendon. An intermediate anchor may be used in this case, e.g., an anchor located at some intermediate point along the tendon used to stress only a portion of the tendon in a completed concrete section leaving a length of remaining tendon free for later post-stressing in a different section. There are many instances where the need arises to post-stress multiple concrete sections using continuous tendons and those multiple concrete sections are being cast sequentially. For example, a parking ramp portion below an office tower (Phase 1) may be built months before an adjoining exterior ramp portion (Phase 2), yet the tendons must be continuous through both portions. The first phase would be stressed, but in many cases this leaves the unused portion of the tendon sitting out exposed for months until the second phase (exterior ramp) can be poured. The exposure to the elements can over time cause the tendon to corrode and lead to early failure.
There are also components used simply to connect two pieces of tendon together. These are called barrel couplers, splice chucks, or in-line stressing couplers. These components join the unsheathed portion of a first tendon to the unsheathed portion of a second tendon by use of internal wedges, springs and other components.
For example, U.S. Pat. No. 6,761,002 to Sorkin (General Technologies, Inc.) issued Jul. 13, 2004 shows a connector assembly for intermediate post-tension anchorage that splices a first tendon to a second tendon with a set of standard wedges 74 (FIG. 2) seated in respective barrel anchors 56, 76 and biased apart by a rubber grommet 104. The wedges 74, barrel anchors 56, 76 and grommet 104 are contained within a stressing barrel 60. The stressing barrel 60 is a sleeve open on one side, closed on the other, with a tendon-passing hole through the closure. One barrel anchor 56 seats into the closed end of stressing barrel 60, and the other barrel anchor 76 screws into the top of barrel 60. The outward-protruding end of barrel anchor 76 seats into intermediate anchor 78 (a standard encapsulated anchor presently sold by General Technologies, Inc. of Stafford, Tex.). An encapsulation sleeve 62 fits overtop and seals around the outside of the anchor 78.
U.S. Pat. No. 6,176,051 to Sorkin (GTI) issued Jan. 23, 2001 shows a splice chuck for use in a post-tension anchor system with a first collar 54 screwed into a threaded end 50 of a body 4, and a second collar 56 is threadedly received within the threaded end 52 of the body 48. The collars 54 and 56 have tapered interiors 58 and 60, respectively. Wedges 62 and 64 are received within the tapered interior 58 of collar 54. Similarly, wedges 66 and 68 are received within the tapered interior 60 of collar 56.
U.S. Pat. No. 6,151,850 to Sorkin (GTI) issued Nov. 28, 2000 shows an intermediate anchorage system utilizing a splice chuck, and a cover 80 (FIG. 2) extending over the splice chuck. The cover 80 has one end in liquid-tight relationship with the tendon, and it extends to a cap that mates with the encapsulation of the intermediate anchor. The cover includes both a polymeric section and an elastomeric portion. The elastomeric portion overlaps an end of the polymeric portion in liquid-tight relationship therewith. The foregoing barrel couplers, splice chucks, or in-line stressing couplers allow shorter lengths of tendons to be installed in phases and joined end-to-end. Then at the next phase or “pour” the concrete can be poured over the tendons and the coupler. Unfortunately, because of the use of threaded collars these prior art barrel couplers, splice chucks, or in-line stressing couplers are difficult to assemble in the field. In addition, they are susceptible to failure and particularly susceptible of corrosion and deterioration. The weakening of any component within the splice chuck can compromise the overall integrity of the splice chuck and, possibly, release the end of one tendon from the end of an adjoining tendon and compromise a joint in the concrete structure.
It would be greatly advantageous to provide a delay anchor that allows the tendon from one phase of construction to be terminated at a joint between a next phase of construction, fully protected from the elements, and then coupled to a remaining portion of the tendon more easily. For this the delay anchor must be simple to assemble in the field, not prone to corrosion or deterioration, and stronger and more robust than prior art devices.